Axial annular swirlers are commonly used to create of vortex flow resulting in a central reverse flow region for stabilization of flames in gas turbine combustors.
FIG. 1 shows a typical swirler arrangement 10. A cylindrical air tube guides an incoming air flow 18 along a longitudinal axis 11 through a swirler section comprising a swirler 14 with a plurality of swirler vanes 19, into a mixing tube 16, where the rotating air flow is mixed with a fuel that is injected by means of fuel injector at the end of a fuel lance 13. The air-fuel mixture then enters a combustion chamber 17 to feed a stabilized flame therein.
Increasing demand on pollution-reduced combustion of conventional fuels as well as hydrogen rich fuels are driving the technical development towards limits of combustion of very lean homogeneously premixed mixtures. The limiting factor in practical combustors is, with the increasing mixture homogeneity, the increasingly strong coupling of the dynamics of the combustion process with the combustor thermoacoustic oscillations.
The stability of the flame, in terms of degree of amplification of the acoustic oscillations, can be improved by optimization of the swirler aerodynamics and the radial profile of the unmixedness of the combustible mixture, entering the flame. Further, the stability and operability of the combustor can be improved by combination of the stabilization by reverse flow, created by the annular swirler with reverse flow in the wake of a bluff body, placed in the centre of the annular swirler.
A pollution-reduced combustion is however not the only demand on the burner. Resistance against flame flash back into the burner along the burner walls is an absolute requirement and low pressure drop of the combustion system, where the swirler can significantly contribute, is important for the gas turbine efficiency.
Document DE 44 06 399 A1 discloses a device for improving fuel-air mixing in re-heat combustors. An annular flow channel of this combustor is limited by a cylindrical interior wall and a cylindrical exterior wall. Both walls are connected by a number of streamlined supports, which are evenly distributed at the circumference and act as guide vanes. The trailing edges of these guide vanes feature a discontinuity, by a notch they are divided into two diverging portions. The radially outer rear half of the guide vane has an uninterrupted profiling of the underpressure surface and the overpressure surface, while the radially inner rear half is directed offset in relation to this, i.e. the profile of the overpressure surface makes a transition into the underpressure surface. By this measure the hot gas flow through the annular passage is split into two diverging partial flows. The vortices generated by the diverging portions of the guide vanes accelerate the mixture of fuel and combustion air and additionally smooth out the concentration and temperature differences in the gas flow.
Document DE 10 2007 004 394 A1 relates to a premixing burner for a gas turbine. In an annular flow channel a swirler for generating a fuel-air-mixture is arranged. The swirler is equipped with streamlined guide vanes. In an inner portion near by the interior wall of the flow channel the trailing edges of these swirler vanes have a recess forming a gap between the airfoil and the interior wall. This discontinuity at the radially inner rear portion supports the generation of tip vortices capable of enhancing premixing.
Document EP 2 233 836 A1 discloses a swirl generator, which has outer wall enclosing central fuel distributor and bounding axial flow channel for combustion air. Swirl vanes extend in radial direction to outer wall to give tangential flow component to flowing combustion air. A separating wall encloses central fuel distributor, and is positioned radially within outer wall to divide flow channel into radially inner channel segment and radially outer channel segment. The radially inner channel segment allows combustion air to pass without giving tangential flow component to combustion air.
Document US 2009/056336 A1 relates to a burner for use in a combustion system of an industrial gas turbine. The burner includes a fuel/air premixer including a splitter vane defining a first, radially inner passage and a second, radially outer passage, the first and second passages each having air flow turning vane portions which impart swirl to the combustion air passing through the premixer. The vane portions in each passage are commonly configured to impart a same swirl direction in each passage. A plurality of splitter vanes may be provided to define three or more annular passages in the premixer.
Document US 2009/183511 A1 discloses a fuel nozzle for a combustor of a gas turbine engine including a nozzle inlet, a combustion area and a swirler disposed between the nozzle inlet and combustion area. The swirler includes a plurality of swirler vanes, each swirler vane capable of creating a pressure difference in fluid flow through the swirler between a pressure side and suction side of the swirler vane. The swirler further includes at least one through airflow hole located in at least one swirler vane of the plurality of swirler vanes. The at least one through airflow hole is capable of utilizing the pressure difference between the pressure side and suction side to promote fluid flow through the at least one airflow hole. Also disclosed is a method for operating a combustor.
Document US 2012/125004 A1 teaches a combustor premixer, which includes a burner tube having a bell mouth-shaped opening, a plurality of tubular bodies telescopically disposed within the burner tube to deliver combustible materials to a premixing passage defined between the burner tube and an outermost one of the plurality of tubular bodies and a plurality of swirler vanes arrayed circumferentially in the opening, each one of the plurality of swirler vanes including a body extending along a radial dimension from the burner tube to the outermost tubular body and a leading edge protruding upstream from the opening.